Open center hydraulic systems generally comprise a fluid power source, such as a fixed displacement pump, having a low-pressure side and a high-pressure side. A reservoir is connected to the low-pressure side to supply the fixed displacement pump with fluid. One or more loads, such as hydraulic cylinders or motors, regulated by open center directional control valves are connected to the high-pressure side to utilize the fluid power generated by the fixed displacement pump. The fluid flow path is from the fixed displacement pump, which draws fluid from the reservoir through any one or more of the open center directional control valves and their associated loads before returning to the reservoir.
The open center directional control valves are typically spool valves having one normally open orifice (NO.sub.1) in parallel with a pair of normally closed orifices (NC.sub.2 and NC.sub.3) that straddle a load. Two directions of flow control through the load (e.g., forward and reverse) require a second pair of normally closed orifices (NC.sub.4 and NC.sub.5). A common spool regulates flow through all of the orifices. One direction of spool movement gradually closes the normally open orifice NO.sub.1 of a center core passage and gradually opens the normally closed orifice NC.sub.2 of a power core passage as well as the normally closed orifice NC.sub.3 of an exhaust core passage. An opposite direction of spool movement also gradually closes the normally open orifice NO.sub.1 of the center core passage while gradually opening the normally closed orifice NC.sub.4 of another power core passage and the normally closed orifice NC.sub.5 of another exhaust core passage. Additional open center spool valves for controlling other loads can be arranged as individual valves or multiple valve sections that are stacked or sandwiched together as a single unit.
The valve spool usually contains longitudinal machined or pressed slots called metering notches which provide for a more gradual opening or closing of the spool orifices. Many different styles of metering notches are incorporated by manufacturers in order to achieve gradual flow control, especially at low flow rates. Some valve spools have metering notches in communication with the normally open orifice NO.sub.1 as well as both normally closed orifices NC.sub.2 and NC.sub.3, and others have metering notches only in communication with the normally open orifice NO.sub.1 of the center core passage and the normally closed orifices NC.sub.3 of the exhaust core orifice. The degree of spool valve overlap also varies from manufacturer to manufacturer.
Each load is connected to a separate branch line controlled by one of the spool valves. With all of the spool valves in the neutral or off position, fluid flows virtually unrestricted from the fixed displacement pump through the normally open orifices NO.sub.1 of the center core passages and back to the reservoir. Shifting the directional valves from neutral toward one direction or the other gradually restricts flows through the normally open orifices NO.sub.1 of the central core passages and pressurizes the power core passages. Further spool movement gradually opens the normally closed orifices NC.sub.2 or NC.sub.4 of the power core passages permitting flows to the loads (e.g., cylinders or motors). Return flows from the loads encounter the normally closed orifices NC.sub.3 and NC.sub.5 of the exhaust core passages, which are also gradually opened by yet further movement of the spool to allow return flows to the reservoir. Since the normally closed orifices NC.sub.3 and NC.sub.5 of the exhaust core passages provide the final restriction of flows returning from the loads to the reservoir, these orifices (NC.sub.3 and NC.sub.5) are primarily responsible for regulating the operating speeds of the loads.
The central core passages of the multiple spool valves are connected in series to the fixed displacement pump, while each of the power core passages of the same spool valves are connected in parallel to the fixed displacement pump. Thus, closing any one of the normally open orifices NO.sub.1 in series creates the potential for pressure in all of the power core passages, including the power core passage whose normally closed orifice NC.sub.2 or NC.sub.4 is gradually opened by further movement of the same spool valve.
At any given setting combination of the spool valves, the pressure in the power core passages, which is substantially equivalent to the output pressure of the fixed displacement pump (i.e., the system pressure), varies as a function of the total flow resistance in the load branches. For example, a maximum system pressure occurs at a given setting combination of the spool valves when the load flow resistance is high enough to force all of the fixed amount of flow from the pump through the normally open orifices NO.sub.1 of the center core passages to the reservoir. Any flow through the load branches (i.e., through either pair of normally closed orifices NC.sub.2, NC.sub.3 or NC.sub.4, NC.sub.5 of the power and exhaust core passages) to the reservoir reduces the potential system pressure from this maximum for the given valve setting combination. A minimum system pressure occurs at the same valve setting combination when the load flow resistance in the load branches is also at a minimum, permitting a maximum flow through the load branches to the reservoir. Predetermined flow rates cannot be established for any given setting of the individual spool valves; because variations in the load flow resistance alters system pressure, which affects the total amount of flow through the branch lines. System pressure and the resulting flow through the branch lines are also affected by variations in the valve settings.
In addition, when two or more spool valves are operated simultaneously, the flow rate through any one load branch can be affected by variations in the load flow resistance of another load branch. At a given system pressure, the load branch exhibiting the relatively decreased load resistance will receive more flow unless its spool valve is returned toward a more neutral position (i.e., "throttled back"). These instabilities make open center hydraulic systems difficult to control. Operators of open center systems often complain of a lack of fine metering control and high forces at valve control levers that further interfere with operator control and contribute to operator fatigue.
Attempts have been made to improve the flow control characteristics of open center valves by reducing variations in the bypass flow rate. See, for example, U.S. Pat. No. 4,139,021 to Ailshie et al. and U.S. Pat. No. 4,178,962 to Tennis. However, neither of these patents address the problem of flow instability caused by concurrently operating loads.